1. (Field of the Invention)
The present invention relates to a fuel control system for an internal combustion engine, and more particularly to a fuel control system for controlling the control lever of a fuel injection pump in an internal combustion engine.
2. (Description of the Prior Art)
Some modern fuel control systems for automotive internal combustion engines do not drive the rack of a fuel injection pump or the valve of a carburetor directly in response to depression of an accelerator pedal, but read the amount of depression of the accelerator pedal as an electric value, and energize a step motor according to the amount of depression expressed by the electric value for controlling the amount of fuel to be supplied to the engine.
FIG. 6 shows a portion of a device for controlling fuel to be supplied to an internal combustion engine, using a step motor. A control lever 5 attached to a governor 3 of an internal combustion engine 4 for a diesel engine is coupled to a step motor 1 by a rod 10. The control lever 5 is operated by the step motor 1, and is normally biased toward its initial position by a return spring 2. The control lever 5 is movable in a range which is limited between an idle stopper 6 and a full-load stopper 7, the range limits of the control lever 5 being detected respectively by an idle position switch 8 and a full-load position switch 9. Although parts associated with an accelerator pedal are not shown in FIG. 6, the amount of depression of the accelerator pedal is normally read by a potentiometer, and the amount of depression (analog quantity) as read by the potentiometer is converted by an analog-to-digital converter to a digital quantity which is used to energize the step motor 1. The number of steps through which the step motor 1 is rotated is controlled linearly in response to the amount of depression of the accelerator pedal. The number of steps is read by a step number counter in an electronic control unit (not shown) in the form of a microcomputer, so that the present position of the step motor is recognized in the electronic control unit at all times.
FIG. 8 is a flowchart of a control sequence of the step motor. The value of the step number counter for the step motor and a target step number are compared with each other, and the following control is effected dependent on the result of comparison:
(1) If the value of the step number counter is smaller than the target step number, then the value of the step number counter is incremented by +1 to drive the step motor through +1 step, and the value of the step number counter and the target step number are compared with each other again.
(2) If the value of the step number counter is greater than the target step number, then the value of the step number counter is decremented by 1 to drive the step motor through -1 step, and the value of the step number counter and the target step number are compared with each other again.
(3) If the value of the step number counter is equal to the target step number, then whether the target step number is 0 or not is checked. If no, then the step motor is held in position. If yes, then control enters an idle process in which the step number counter is cleared to 0, and the step motor is de-energized.
In such fuel control system for an internal combustion engine, the potentiometer is employed to detect the amount of depression of the accelerator pedal. However, the detected output of the sensor suffers errors due to irregular resistance characteristics of individual potentiometers, and hence the amounts of fuel supplied to internal combustion engines become uneven. Japanese Laid-Open Patent Publication No. 60-11642, as referred to above, proposes a system for preventing variations in the amount of supplied fuel due to irregular resistance characteristices of potentiometers.
The disclosed fuel control system for internal combustion engines has however not taken into consideration any correction of variations in the amounts of fuel supplied by individual fuel supply devices. More specifically, such a problem will be described with reference to a diesel engine, for example. As shown in FIG. 9, the number of steps between idle and full-load positions for a fuel injection pump varies from pump to pump. Since however the corrected number of steps issued from the analog-to-digital converter with respect to the depressed position of the accelerator pedal is solely determined as shown in FIG. 7, different power outputs may be produced from different internal combustion engines even when their accelerator pedals are depressed to the same position. Take, for example, two governors having different angles between idle and full-load positions, with the numbers of steps between the idle and full-load positions being expressed respectively by S1, S2. For controlling the governors under a 1/2 load, the required angle COM of rotation of the step motor is S1/2 for the former governor and S2/2 for the latter governor. This holds true for a fuel supply device for an internal combustion engine using gasoline as fuel.
Accordingly, it is an object of the present invention to provide a fuel control system for an internal combustion engine, which is capable of controlling a step motor so that the same engine output will be produced in response to the same accelerator pedal position even if different angles are present between idle and full-load positions.
Another object of the present invention is to provide a fuel control system for an internal combustion engine, which is capable of preventing a step motor that drives a fuel supply device from a step-out condition.
Still another object of the present invention is to provide a fuel control system for an internal combustion engine, which prevents the internal combustion engine from running out of control even when the fuel control system fails.